The present invention relates to pre-charging of battery devices and powerlines, and more specifically to a self-protecting, pre-charge circuit.
Pre-charge of the powerline voltages in a high a DC voltage application is a preliminary mode in which current from the power source is limited such that a controlled rise time of the output voltage during power up is achieved. Pre-charging reduces the electrical hazards which may occur when the system integrity is compromised due to hardware damage or failure.
Pre-charging is commonly used in high voltage battery charging applications. For example, a conventional battery charging system typically includes a high voltage/high current charger (i.e., the power supply), a battery management system circuit (i.e., a control circuit, contactors, capacitors, resistors, etc), and load circuits (i.e., inductors, capacitors and resistors) coupled to the battery. The pre-charge of a high voltage DC bus is necessary in order to slowly (0.1 s to a few seconds) bring the load voltage from 0V to a target voltage (usually, hundreds of volts) in order to be able to close the main high voltage contactors without causing an on rush of current. Such a rush of current can weld such contactors closed and potentially damage the battery or other components in the system.
In general, the energy to perform this pre-charge operation in battery charging systems is typically controlled by a pre-charge circuit that limits the amount of current present on the high voltage bus. In most conventional charging systems, this pre-charge circuit is implemented as a series of contactors or switching elements for selecting between a first current path including a current limiting resistor (the pre-charge resistor) and a second, low resistance path. Thus, during the pre-charge process, the first path is used to limit the amount of current delivered to the high voltage bus. Once pre-charged, the pre-charge circuit can switch to the low resistance path to complete the charging process.
However, in many cases the actual energy in the charging system can be dependent on many factors other than the power supply. For example, there can be unintended high voltage loads or capacitances present in the system. As a result, the is a strong likelihood of the occurrence of a large flow of current that could potentially damage not only the contactors in the system, by also other components, such as the pre-charge hardware or the battery itself. For example, some types of pre-charge resistors can be susceptible to damage from such large currents.